(1) Field of the Invention
The present invention relates to a developing process in the electrophotography. More particularly, the present invention relates to a developing process for forming a toner image having a high quality.
(2) Description of the Related Art
In the electrophotography using a two-component type magnetic developer, an electroscopic toner is mixed with a magnetic carrier, and this two-component composition is supplied to a developing sleeve having magnets disposed in the interior thereof to form a magnetic brush composed of this composition. An electrophotographic photosensitive plate having an electrostatic latent image is brought into contact with this magnetic brush to form an electroscopic toner image on the photosensitive plate. The electroscopic toner is charged with a polarity reverse to the polarity of the electrostatic latent image on the photosensitive plate by the friction with the magnetic carrier, and electroscopic toner particles on the magnetic brush are caused to adhere onto the electrostatic latent image by the Coulomb force to effect the development of the electrostatic latent image. On the other hand, the magnetic carrier is attracted by the magnets arranged in the sleeve, and the charged polarity of the magnetic carrier is the same as the polarity of the electrostatic latent image. Accordingly, the magnetic carrier is left on the sleeve.
Japanese Unexamined Patent Publication No. 59-172660 proposes a process in which a two-component type developer comprising a ferrite carrier and chargeable toner particles at a weight ratio of from 4/1 to 20/1 is used as the two-component type developer, a developing sleeve is moved in a direction opposite to the moving direction of a drum at the position of the sliding contact between the drum and the developer, and the development is carried out while fixing magnets in the sleeve under conditions satisfying the following requirements: ##EQU1## wherein .theta. represents the angle (degrees) of setting the magnetic pole closest to the line connecting the center of the drum and the center of the sleeve in the downstream direction of the rotation of the drum, V represents the peripheral speed of the drum, and .upsilon. represents the peripheral speed of the sleeve.
It is deemed that this conventional technique is significant in that by setting the peripheral speed of the drum, the peripheral speed of the sleeve and the angle of the main developing pole within certain ranges, a high-density image free of such defects as formation of brush marks, tailing of the carrier and formation of a blank portion by the edge effect can be obtained. In this process, however, problems arise because of the fact that the moving direction of the developing sleeve is reversed to the moving direction of the drum at the position of the sliding contact (this sliding contact will be called "reverse direction sliding contact" hereinafter, and the sliding contact in the state where both of the sleeve and drum are moved in the same direction will be called "forward direction sliding contact" hereinafter).
In the reverse direction sliding contact, the relative speed is higher than in the forward direction sliding contact, and therefore, formation of brush marks is more conspicuous than in the forward direction sliding contact, with the result that the reproducibility of fine lines is reduced and an image of a hard tone is often formed. Therefore, reduction of the reproducibility of Chinese characters in a word processor or the like cannot be avoided.
Accordingly, in recent commercial electrophotographic processes, the forward direction sliding contact is often adopted for the development. In case of the forward direction sliding contact, however, the relative speed for the sliding contact is inevitably low, and it is not easy to set such developing conditions that both of the image density and the reproducibility of fine lines can be simultaneously improved.